(1) Field of the Invention
This invention relates to a process for producing a Y-type zeolite which possesses a faujasite-type crystal structure and a high SiO.sub.2 /Al.sub.2 O.sub.3 ratio.
(2) Description of the Prior Art
A Y-type zeolite possessing a faujasite-type crystal structure is generally represented by the formula: EQU 0.9.+-.0.2M.sub.2 O.Al.sub.2 O.sub.3.wSiO.sub.2.zH.sub.2 O
wherein M is an alkali metal, w is a number in the range of from 3 to 7 and z is a number in the range of from 0 to 9. Such a Y-type zeolite is generally produced by a process wherein an alumina (Al.sub.2 O.sub.3) source, such as sodium aluminate, active alumina or gamma alumina, and a silica (SiO.sub.2) source, such as silicic acid, sodium silicate, silica gel or white carbon, are reacted with each other in an aqueous reaction medium in the presence of an alkali, and then, the crystalline particles of zeolite, so formed, are separated from the reaction medium.
Y-type zeolites are popularly used as catalysts for, for example, cracking of petroleum, alkylation of hydrocarbons and isomerization of n-paraffins and as adsorbents for, for example, separation of p-xylene. It is very desirable for Y-type zeolite catalysts and adsorbents to possess good thermal resistance and acid resistance. In general, the thermal resistance and the acid resistance of Y-type zeolites are closely related to the SiO.sub.2 /Al.sub.2 O.sub.3 ratio thereof. The higher the SiO.sub.2 /Al.sub.2 O.sub.3 ratio, the better the thermal resistance and the acid resistance. Y-type zeolites having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of below about 4 do not exhibit thermal and acid resistances to a practically permissible extent. Thus, Y-type zeolites having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of at least about 4 are useful. Although it is desirable for Y-type zeolites to have a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of about 5 or more, it is difficult to produce such Y-type zeolites with good reproducibility.
Japanese Patent Publication No. 1,639/1961 teaches that, although conventional synthetic Y-type zeolites prepared by using sodium silicate as a silica source have a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of below 3.9, synthetic Y-type zeolites prepared by using a solid silica, such as silica sol or white carbon, have a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of at least about 4. However, a solid silica such as silica sol or white carbon is expensive.